Diamond-containing cemented metal carbide

ABSTRACT

Cemented tungsten carbide rock bit inserts have diamond particles dispersed therein for enhanced hardness and wear resistance. The cobalt matrix is primarily the face centered cubic crystal structure for enhanced ductility and toughness. Such inserts are formed by introducing excess non-diamond carbon beyond the stoichiometric proportion in the tungsten carbide. The inserts are pressed at a temperature and pressure where diamond is thermodynamically stable and cooled while maintaining the pressure sufficiently high to prevent decomposition of diamond crystals formed at the elevated temperature. Diamond crystals may be dispersed throughout the cemented tungsten carbon or may be more concentrated near the surface than in the interior.

FIELD OF THE INVENTION

This invention relates to cemented tungsten carbide articles havingdiamond particles formed in situ in the metal binder for tungstencarbide particles. In particular, it relates to techniques for makingcemented tungsten carbide inserts for rock bits with diamond particlesdispersed in the matrix.

BACKGROUND OF THE INVENTION

Oil wells and the like are commonly drilled with rock bits having rotarycones with cemented tungsten carbide inserts. As such a bit is rotatedon the bottom of a drill string in a well, the cones rotate and thecarbide inserts bear against the rock formation, crushing and chippingthe rock for extending the depth of the hole. Typical inserts have acylindrical body which is pressed into a hole in such a cone and asomewhat blunt converging end that protrudes from the face of the cone.The converging end of the insert may be generally conical, roughlyhemispherical, or have a somewhat chisel-like shape. Another type of bitfor drilling rock employs a steel body in which similar tungsten carbideinserts are embedded. The bit is hammered against the bottom of the holefor shattering rock and gradually rotated as it drills. Inserts providedin practice of this invention may be used in either type of rock bit, orin other related devices such as underreamers.

Since the tungsten carbide inserts are the parts of the rock bit thatengage and drill the rock, it is important to minimize wear and breakageof such inserts. Tungsten carbide inserts for rock bits are made bysintering a mixture of tungsten carbide (WC) powder and cobalt to form adense body with very little porosity. Two important properties of suchinserts are wear resistance and toughness. It is desirable to enhancethe hardness of an insert adjacent to its surface where it engages therock formation and maintain toughness for minimizing breakage of theinsert as it is used.

In rock bits designed for a particular type of service, one needs tohave an appropriate balance between hardness and toughness. Hard insertsresist wear during drilling. On the other hand, a hard insert may besusceptible to fracture under the impact loads and other abusesnecessarily involved in drilling wells. Enhanced toughness is alsoadvantageous, since the part of the insert extending beyond the face ofthe cone does not need to be as blunt to resist fracture. This meansthat a longer, more aggressive cutting structure can be employed on arock bit where fracture toughness is adequate. Thus, soft formation bitsmay have longer insert protrusion than bits intended for use on harderrock formations.

In essentially all bits, it is desirable to have high hardness and wearresistance and relatively large insert protrusion. Achievement of thesedesiderata may, however, be limited by a lack of fracture toughness inthe main body of the insert. It is desirable to have a hard surface anda tough body. Of course, hardness and toughness throughout the insert isalso desirable.

Composite rock bit inserts have been made comprising a layer ofpolycrystalline diamond on the protruding, converging end of a cementedtungsten carbide insert. This provides a high hardness at the surfaceand a tough body within the insert. There are appreciable differences inthe mechanical and thermal expansion properties of such apolycrystalline diamond layer and the underlying cemented tungstencarbide. A transition layer, comprising a mixture of carbide and diamondcrystals, has, therefore, been provided between the polycrystallinediamond layer and the principal body of the carbide insert. Such aninsert is shown, for example, in U.S. Pat. No. 4,694,918.

Such an insert may be made by forming a layer of diamond crystals mixedwith a small amount of cobalt. Over this there is formed a layercontaining a mixture of diamond crystals and precemented tungstencarbide particles. One or more additional layers containing a differentproportion of carbide and diamond may be added. A cemented tungstencarbide blank is then placed on the final layer having a mixture ofcarbide and diamond. This entire assembly is then placed in a very highpressure press and subjected to sufficiently high pressure and elevatedtemperature to be in a region where diamond is thermodynamically stable.Exemplary minimum temperature is about 1200° C. and an exemplary minimumpressure is about 40 to 45 kilobars.

The assembly is heated and cooled under elevated pressure. This resultsin formation of a layer of polycrystalline diamond tightly adherent tothe cemented tungsten carbide, with one or more transition layersbetween the polycrystalline diamond and the cemented tungsten carbidebody. Such transition layers are a mixture of diamond crystals andprecemented tungsten carbide. The polycrystalline diamond layer has highhardness. The cemented tungsten carbide main body of the insert has goodtoughness. The transition layer or layers help accommodate thedifferences in thermal expansion and mechanical properties between thepolycrystalline diamond and the cemented tungsten carbide.

It is desirable to provide other techniques for forming a transitionlayer for a rock bit insert having a polycrystalline diamond surface. Itis also desirable to provide a rock bit insert having diamond particlesdistributed in a matrix of tungsten carbide and cobalt. It is alsodesirable to enhance the wear resistance of a cemented tungsten carbiderock bit insert without significantly degrading toughness.

BRIEF SUMMARY OF THE INVENTION

There is, therefore, provided in practice of this invention, accordingto a presently preferred embodiment, a method for forming a cementedtungsten carbide article with embedded diamond particles by introducingexcess carbon beyond the stoichiometric proportion of tungsten carbideinto a mixture of tungsten carbide particles and cobalt. The mixture ispressed into the shape of the desired article and sintered. The sinteredpart is then pressed at a temperature and pressure where diamond isthermodynamically stable, and it is cooled out of the thermodynamicallystable region while maintaining sufficient pressure to preventdecomposition of diamond.

If a uniform distribution of diamond particles is desired throughout thearticle, excess carbonaceous material such as wax or graphite may bemixed with the carbide and cobalt before pressing. If it is desired tohave a higher proportion of diamond particles near the surface and asmaller proportion of diamond in the core of such an article, thearticle may be carburized and then subjected to elevated temperature andpressure to form diamonds in situ within the matrix for the carbideparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description and when considered in connection withthe accompanying drawings, wherein:

FIG. 1 illustrates a typical, conventional rock bit in which insertsmade in practice of this invention are employed;

FIG. 2 illustrates an exemplary insert in longitudinal cross section;and

FIG. 3 illustrates, in longitudinal cross section another example of arock bit insert made in practice of this invention.

DETAILED DESCRIPTION

Oil and gas wells and the like are commonly drilled with so called threecone rock bits. Such a rock bit has a steel body 20 with threads 14 atits upper end and three depending legs 22 at its lower end. Three steelcutter cones 16 are rotatably mounted on the three legs at the lower endof the bit body. A plurality of cemented tungsten carbide inserts 18 arepress-fitted into holes in the surfaces of the cones. Lubricant isprovided to the journals on which the cones are mounted from each ofthree grease reservoirs 24 in the body.

When the rock bit is used, it is threaded onto the lower end of a drillstring and lowered into a well. The bit is rotated with the carbideinserts in the cones engaging the bottom of the hole. As the bitrotates, the cones rotate on the body, and essentially roll around thebottom of the hole. The weight on the bit is applied to the rockformation by the carbide inserts and the rock is thereby crushed andchipped by the inserts. A drilling mud is pumped down the drill stringto the bottom of the hole and ejected from the bit body through nozzles26. The mud then travels up the annulus between the drill string and thehole wall. The drilling mud provides cooling and removes chips from thebore hole.

Improved inserts provided in practice of this invention may be made byconventional techniques in the first part of the processing. Thus, amixture of tungsten carbide powder and cobalt powder is milled with atemporary wax binder. The mixture is pressed to form a "green" compacthaving the same shape as the completed insert. This shape is in the formof a cylinder 28 with a converging end portion 30 at one end of thecylinder. The converging portion may have any of a number ofconventional configurations, including a chisel-like end, ahemispherical end, or a rounded conical end.

The green compacts are loaded into a high temperature vacuum furnace andgradually heated until the temporary binder wax has been vaporized. Thetemperature is then elevated to about the melting temperature of thecobalt, whereupon the compact is sintered to form an insert of highdensity, that is, without substantial porosity. The inserts are thenrelatively slowly cooled in the vacuum furnace. After tumbling,inspection and grinding of the cylindrical body, such inserts have beenused for many years in rock bits.

Such a pressed and sintered insert may be used in practice of thisinvention. In such an embodiment, the sintered insert is carburized in aconventional manner. Either pack, gas, or liquid carburizing may beused. Carburizing involves holding the insert at elevated temperature inan environment with a high carbon pressure so that carbon is introducedthrough the surface of the insert. Such carbon diffuses into the insertthrough the cobalt phase serving as a matrix for the tungsten carbideparticles. The carbon concentration in the carburized insert, the depthof the carburized material, and the profile of carbon concentration as afunction of depth are functions of the time and temperature ofcarburizing, the composition of the carburizing environment, and thecobalt content of the cemented carbide.

Carburizing sintered tungsten carbide is generally accomplished bypacking inserts in a bed of graphite powder and heating in ahydrogen/inert gas mixture or in vacuum. The carburizing introducesexcess carbon into the cemented tungsten carbide insert in excess of thestoichiometric proportion of tungsten carbide. Other techniques forcarburizing are thoroughly described in Metals Handbook, 8th Ed., Vol.2, American Society for Metals, 1964.

It may be desirable to carburize only the converging end portion of theinsert. This may be preferable since, as described hereinafter, theexcess carbon is converted to diamond crystals dispersed in the cementedtungsten carbide. Typically, the cylindrical surface of the principalportion of the insert is ground to its final desired dimension as thelast step of the process. Although diamond grinding is used, it ispreferable to avoid a large proportion of diamond in the cylindricalportion to make grinding easier. To minimize carburization in thisregion, a conventional "stop off" may be painted on the surface or thesurface may be plated with a carbon-resistant material such as copper,as is conventional and well known in the carburizing art.

After carburizing, the cemented tungsten carbide insert is placed in theworking volume of a super-pressure press of the type used forsynthesizing diamond crystals. A tetrahedral press, cubic press, or beltpress is suitable. A technique for pressing the insert is described inU.S. Pat. No. 4,694,918.

The insert, with or without a protective metal can, is surrounded bypyrophyllite or salt so that it is subjected to isostatic pressure.Sufficient pressure is then applied that diamond is thermodynamicallystable at the temperatures involved in the process. In an exemplaryembodiment, a pressure of 60 kilobars is used. A minimum pressure ofabout 40 to 45 kilobars should be used. As soon as the assemblycontaining the insert is at high pressure, current is passed through agraphite heater tube within the press to raise the temperature to atleast 1200° C., and preferably, to about 1400° C. Such pressure andtemperature are held for 60 seconds so that diamond particles form fromthe excess carbon in the cobalt phase.

The current is then turned off, and the parts rapidly cool by heattransfer to the water cooled anvils of the press. When the temperatureis below about 800° C., and preferably below 200° C., the pressure canbe released so that the material in the working volume can be ejectedfrom the press. The insert may then be finished for use by diamondgrinding the cylindrical body.

As a result of this treatment, a dispersion of particles forms in thecobalt matrix, which X-ray diffraction confirms includes diamond. It iscertain that very hard particles are present in the matrix, andmetallographic specimens are harder to polish than ordinary cementedtungsten carbide. Further, graphite inclusions, which commonly occur incemented tungsten carbide specimens with excess carbon, are no longerpresent. It appears that there is complete conversion of excess carbonto diamond, except for some residual carbon that may remain in solutionin the cobalt matrix.

An insert which has been carburized and treated at high pressure andtemperature to form dispersed diamond crystals from the excess carbonmay be used in a rock bit where the converging portion of the insertprotrudes from the cone and engages the rock formation being drilled.Enhanced wear resistance due to diamonds in the cobalt matrix is useful.

Alternatively, such an insert may be the substrate on which a layer ofpolycrystalline diamond is formed. The carburized and transformed layerthen forms a transition between a layer of polycrystalline diamond 32and the principal body 34 of the insert. In such an embodiment, diamondcrystals and about 6 per cent by volume of cobalt powder are ball milledtogether. The blended diamond powder and cobalt may then be placed in azirconium cup or the like having an internal shape corresponding to theshape of the desired insert. The powder is spread into a thin layer byrotation and pressing with an object having the same shape as the insertwhen it is axisymmetric. The insert itself can be used for spreading thepowder. The insert is put in place over the layer of powder and the endof the cup may then be closed with a zirconium disk.

Alternatively, a mixture of diamond powder and cobalt may be blendedwith a wax and formed into a thin cap for the insert. This assembly isthen placed in a super-pressure press and processed in the same manneras hereinabove described. The high pressure and high temperature causethe layer of diamond powder to be formed into a layer of polycrystallinediamond tightly adherent to the cemented tungsten carbide of the insert.The formation of the polycrystalline diamond layer and creation ofdiamond particles dispersed in the cobalt matrix can be accomplishedsimultaneously in a single high pressure, high temperature cycle byplacing the carburized insert in the press. Alternatively, the diamondparticles may be formed in the matrix first, although no advantage totwo cycles through the press has been noted.

In still another embodiment, it may be desirable to have diamondparticles dispersed throughout the body of cemented tungsten carbide. Inthat case, the insert may be fabricated from a mixture of powders ofgraphite, tungsten carbide and cobalt. These powders are mixed, pressedand sintered as hereinabove described.

Alternatively, tungsten carbide and cobalt particles may be mixed with acarbonaceous wax or the like which decomposes to leave a carbonaceousresidue, rather than vaporizing during the sintering process. In stillanother alternative, the original compact may be sintered in acarbonaceous environment, in which case an excess of carbon is obtainedmore or less throughout the insert. In any of these arrangements, excesscarbon beyond the stoichiometric proportion of tungsten carbide ispresent in the sintered product.

Preferably the amount of excess carbon is in the range of from two tofifteen percent by volume of the composite cemented tungsten carbide.Lower carbon proportions are suitable where the proportion of cobaltbinder is low, however, less than about two percent by volume shouldshow such a small benefit that the added cost of processing is notjustified.

Concomitantly, higher proportions of carbon are used when the cobaltcontent is higher. Generally speaking, it is desirable to employ a highcobalt content for enhanced toughness and resistance to breakage. Theconversion of carbon to diamond in a higher cobalt composite enhancesthe wear resistance to offset the usual decrease in wear resistance ofhigher cobalt grades of cemented carbide. More than about fifteenpercent by volume of graphite is undesirable since decreases intoughness may be observed.

In applications where the composite cemented carbide article withdiamond particles dispersed in the matrix is to be used as a cutting ormachining tool, higher proportions of excess carbon may be useful.Graphite contents up to 50% by volume may be employed where the tungstencarbide content of the composite is concomitantly reduced. Sufficientcobalt should be present for catalyzing substantially completeconversion of graphite to diamond. If the carbon content is too high,cracking of the composite article may be observed due to differentialthermal expansion or excessive shrinkage.

When such a cemented tungsten carbide article with excess carbon is thenprocessed in a super-pressure press at temperatures and pressures wherediamond is thermodynamically stable, excess carbon in the article isconverted to diamond particles dispersed through the cobalt phase.

In addition to the hardening due to presence of very high hardnessdiamond particles, it appears that such a dispersion of diamondparticles has a dispersion hardening effect on the cobalt, resulting ina harder and stronger insert. The enhanced hardness of the cobalt phaseby dispersion hardening enhances wear resistance of the inserts. It isbelieved that wear of cemented tungsten carbide occurs, at least inpart, by reason of extrusion of cobalt from between carbide grains,thereby exposing cobalt at the surface where it is subject to wear.Absence of ductile cobalt between carbide grains may then contribute towear resistance of the tungsten carbide.

In addition, the temperature to which the insert is heated in thesuper-pressure press is sufficient to transform hexagonal close packedcobalt to a face centered cubic crystal structure which is stable at theelevated temperature and pressure. Rapid cooling in the press from thehigher temperature through the phase transformation temperature retainsprimarily metastable face centered cubic crystal structure in the cobaltmatrix.

This is desirable since the face centered cubic structure issubstantially more ductile than the hexagonal close packed crystalstructure, thereby imparting enhanced toughness to the rock bit insert.Such enhanced toughness is desirable for minimizing susceptibility tobreakage of the insert as it is used in a rock bit.

EXAMPLE

Cemented tungsten carbide specimens were made with tungsten carbideparticles having an average particle size in the order of 2.5micrometers. Tungsten carbide powder was mixed with 11 per cent byweight cobalt powder. Three types of specimens were made from such amixture. The first type was sintered from such a mixture. The secondtype had an excess of ten per cent by volume of added carbon. The thirdtype was the same as the second, but was then processed at hightemperature and pressure for converting carbon to diamond. The insertswere pressed at 60 kilobars at 1400° C. for about 60 seconds in salt,without a sheath. The following table compares the properties of thesethree types of specimens.

    ______________________________________                                              Hardness Density  Coercivity                                                                            Mag    Wear Scar                              Sample                                                                              Ra       g/cc     Oe      Sat %  Depth, mm                              ______________________________________                                        1     88.4     14.41    90      92     0.69                                   2     85.6     12.97    104     92     1.38                                   3     88.9     13.56    84      85     0.53                                   ______________________________________                                    

The hardness and density decreases between samples 1 and 2 are due tothe presence of graphite inclusions in the cemented tungsten carbide.The increased coercivity is probably due to stabilization of thehexagonal close packed crystal structure as a result of increased carboncontent and decreased solubility of tungsten in the cobalt phase. Thesubstantially poorer wear resistance is to be expected.

Sample 3 after processing at high temperature and pressure has ahardness and wear resistance greater than that of Sample 1, which is thecemented tungsten carbide without excess carbon. This increase inhardness and wear resistance is consistent with dispersion hardening ofthe cobalt phase by dispersed diamond particles. The intermediatedensity of Sample 3 between the densities of Samples 1 and 2, is alsoconsistent with formation of diamond crystals, which are more dense thanthe graphite. It could be noted, however, that the very high pressuresand temperatures could also reduce some inherent porosity in thecemented tungsten carbide following the original processing.

The substantial decreases in coercivity and magnetic saturation areindicative of stabilization of the face centered cubic crystal structurein Sample 3 after high pressure and high temperature processing.

Significantly, Sample 3 which was processed at high temperature andpressure, shows wear resistance better than that of the originalcemented tungsten carbide. It should be noted that in the tests set outin the table, the contact area during the wear test of Sample 1 wasgreater than the contact area of Sample 3. This means that there wasless force per unit area applied to the specimen. Even so, the untreatedcemented tungsten carbide showed more wear than specimens treated athigh temperature and pressure for forming a dispersion of diamondcrystals. There is an increase in abrasion resistance of at least 25%,which makes the material particularly suitable for rock bit inserts andfor cutting tools for machining metals and abrasive composites.

Metallographic examination showed that the assintered inserts of Sample2 had voids where graphite inclusions were pulled out during polishing.Sample 3, which was processed at high temperature and pressure, has harddiamond particles embedded in the matrix, some as large as tenmicrometers.

Although limited embodiments of cemented metal carbide articlesstrengthened or hardened by in situ formed diamond particles dispersedin the matrix have been described herein, many modifications andvariations will be apparent to those skilled in the art. Thus, forexample, the metal carbide is not necessarily entirely tungsten carbide.Tantalum carbide or titanium carbide may be present as well. Thus, forexample, where such an article is to be used as a cutting tool or thelike, it is desirable to employ titanium carbide.

Iron group metals other than cobalt may also be employed. The iron groupmetals, such as iron and nickel, are suitable for cementing tungstencarbide particles and may also act as catalyst for synthesis of diamondcrystals. Many other modifications and variations will be apparent tothose skilled in the art and it is, therefore, to be understood thatwithin the scope of the appended claims the invention may be practicedotherwise than as specifically described.

What is claimed is:
 1. A method for forming a cemented tungsten carbidearticle with embedded diamond particles comprising the stepsof:introducing excess non-diamond carbon beyond the stoichiometricproportion in tungsten carbide into a cemented tungsten carbide article;pressing the article at a temperature and pressure where diamond isthermodynamically stable; and cooling the article out of thethermodynamically stable region while maintaining the pressuresufficiently high to prevent decomposition of diamond.
 2. A method asrecited in claim 1 wherein the introducing step comprises:mixingtungsten carbide particles, cobalt and non-diamond carbonaceousmaterial; and sintering the mixture into the shape of the article beforepressing.
 3. A method as recited in claim 2 wherein the carbonaceousmaterial comprises a wax.
 4. A method as recited in claim 2 wherein thecarbonaceous material comprises graphite.
 5. A method as recited inclaim 1 wherein the introducing step comprises:forming a cementedtungsten carbide article; and carburizing the article before thepressing step.
 6. A method as recited in claim 5 wherein the carburizingstep comprises carburizing the article during the forming step.
 7. Amethod as recited in claim 1 wherein the excess carbon content is in therange of from two to fifteen percent by volume of the cobalt.
 8. Amethod as recited in claim 1 further comprising placing a layercontaining diamond crystals adjacent to a surface of the article beforethe pressing step so that an adherent layer of polycrystalline diamondis formed on the surface of the article upon pressing and cooling.
 9. Amethod for forming a cemented metal carbide article with embeddeddiamond particles comprising the steps of:introducing excess non-diamondcarbon beyond the stoichiometric proportion in the metal carbide into acemented composite article comprising metal carbide particles and irongroup metal; and subjecting the article to diamond making conditions ofhigh pressure and temperature.
 10. A method as recited in claim 9wherein the introducing step comprises:mixing metal carbide particles,iron group metal and non-diamond carbonaceous material; and sinteringthe mixture into the shape of the article before pressing.
 11. A methodas recited in claim 10 wherein the carbonaceous material comprisesgraphite.
 12. A method as recited in claim 9 wherein the introducingstep comprises:forming a cemented metal carbide article; and carburizingthe article before the pressing step.
 13. A method as recited in claim 9wherein the excess carbon content is in the range of from two to fifteenpercent by volume of the iron group metal.
 14. A method as recited inclaim 9 wherein the excess carbon content is up to fifty percent byvolume of the composite article.
 15. A method for forming an insert fora rock bit comprising the steps of:forming a mixture of tungsten carbideand cobalt particles into a rock bit insert; including in the insert anexcess of non-diamond carbon greater than the stoichiometric proportionin the tungsten carbide; and pressing the insert at a sufficienttemperature and pressure to form diamond from excess carbon.
 16. Amethod as recited in claim 15 comprising:mixing tungsten carbideparticles, cobalt and graphite; and sintering the mixture beforepressing.
 17. A method as recited in claim 15 comprising forming thecemented tungsten carbide insert and carburizing the insert before thepressing step.